1. Field of the Invention
The invention relates to a method and an apparatus for determining a mobility of ions. The method includes the steps of modulating an ion beam with an ion gate which is controlled by a modulation function for generating a modulated ion beam, of guiding the modulated ion beam through a drifting region, of measuring a signal of the modulated ion beam after the modulated ion beam has passed the drifting region and of calculating a correlation of the modulation function and the signal in order to determine the mobility of the ions. The apparatus includes the ion gate, the drifting region through which the modulated ion beam is guidable, a detector by which the signal of the modulated ion beam is measurable after the modulated ion beam has passed the drifting region and a calculation unit by which the correlation of the modulation function and the signal is calculable in order to determine the mobility of the ions.
2. Description of the Related Art
Methods and apparatuses pertaining to the above mentioned technical field are known. For example, in US 2009/0294647 A1 (Karsten Michelmann), an ion mobility spectrometer which is coupled to a mass spectrometer and a corresponding measuring method are described. The ion beam is modulated with a continuous modulation function and the modulation frequency of this modulation function is varied over a large frequency range. In order to obtain the ion mass spectrum, the measured ion spectrum is correlated with the modulation function of the ion beam.
Another ion mobility spectrometer and a corresponding method are disclosed in U.S. Pat. No. 7,417,222 B1 (Sandia Corp). There as well, the ion beam is modulated with a modulation function and the measured signal is correlated with the modulation function. But in contrast to US 2009/0294647 A1, the modulation function may also be a binary function. In particular, Barker codes are described as being favorable modulation functions because their autocorrelation provides low side bands.
A somewhat different approach is described in U.S. Pat. No. 6,900,431 B1 (Predicant Biosciences, Inc.) on the example of a time of flight mass spectrometer. Here, the ion beam is modulated in pseudo random sequences of maximum length. The characterization of the ion spectra is obtained by the inverse Hadamard transformation formalism. Similarly, in the ion mobility spectrometer disclosed in WO 2004/097394 A1 (Smiths Group Plc), the ion beam is modulated with a pseudo random sequence of maximum length and the measured ion signal is analyzed by a matrix algebra. But in the latter example, a Fourier analysis may be used instead of the matrix algebra, too. Additionally, two modulation sequences with inverted bits may be used in order to obtain a better signal to noise ratio.
These known methods have in common that the ion beam is modulated according to a modulation function, that the ion signal is measured after the ions have passed a drifting region and that the ion mobility is obtained by calculating a correlation of the modulation function with the measured ion signal. This procedure for obtaining the ion mobility is employed because it is not required to know the starting time of each individual ion as it would be if directly measuring the ion's flight time. Consequently, it is possible to pass at the same time more than one pulse or packet of ions through the drifting region. This has the advantage that more ions can be measured within the same period of time.
The disadvantage of this procedure is that calculating the correlation introduces features into the ion mobility spectra which cannot easily be identified as such. For example, these features may be small peaks in the ion mobility spectra that look like a signal obtained from some specific ion species. Therefore, if traces of ions are to be detected, such artificially introduced features are likely to lead to misinterpretations of the ion mobility spectra. Thus, in order to avoid such misinterpretations, small peaks in the ion mobility spectra have to be discarded as possible false peaks. This significantly limits the attainable dynamic range.